1. Field of the Invention
The present invention relates to a method of removing metal ion containing impurities from wet process phosphoric acid. More particularly, the present invention relates to a process of precipitating magnesium and other metal ion impurities from wet process phosphoric acid by ammoniation.
2. Description of the Prior Art
Wet process phosphoric acid is made by the acidulation of phosphate rock with sulfuric acid. Gypsum is the main by-product of the reaction. Phosphate rock contains many different types and amounts of mineral impurities and these impurities are solubilized by the process resulting in incorporation of the impurities in the product acid. Because the quality of phosphate rock is declining as the better grades of the mineral are being removed by mining, the impurity levels in the wet process acid being obtained from this rock have been increasing.
The presence of impurities in wet process phosphoric acid results in a variety of problems for those who manufacture N-P and N-P-K fertilizers. One problem is that the presence of the impurities dilutes the nitrogen, P.sub.2 O.sub.5 and potassium contents of the fertilizer thus resulting in a lower grade fertilizer product. Impurities precipitate from the phosphoric acid and settle from solution during storage of the acid thereby resulting in sludge accumulation in storage tanks, transfer lines and other apparatus sections. Some of the impurities present in the acid such as aluminum salts actually make the wet process acid more difficult to concentrate. Still further, some impurities in the acid, despite processing of the acid for removal of the impurities, are not removed from the acid and appear in liquid fertilizer products where they then can precipitate resulting in (1) a build-up of solids in storage tanks which effectively reduces storage capacity and results in reprocessing and cleaning costs; (2) the gradual formation of precipitated solid matter in transportation equipment such as barges, railcars and the like causing product loss and equipment cleansing costs; (3) the clogging of liquid fertilizer application equipment, particularly orifices through which the fertilizer is applied; (4) solids which tie up P.sub.2 O.sub.5 in a form that is unavailable to crops (i.e., citrate insoluble form); (5) general dissatisfaction by the consumer of the product.
Because of the desirability of removing impurities from wet process acid, a number of different impurity removal methods have been developed. One such method is solvent extraction, of which there are a number of variations developed to the point of commercial availability. Basically, solvent extraction involves the extraction of either P.sub.2 O.sub.5 or impurities from wet process acid, while leaving other components behind in the aqueous phase. Significant disadvantages of solvent extraction are the high capital and operating costs and the fact that organic solvents must be handled.
A second type of impurity removal process is concentration/clarification of wet process phosphoric acid. In the first step of the process, wet process acid is concentrated to 70% P.sub.2 O.sub.5 (super acid) and some of the impurities are allowed to precipitate and settle from the concentrate. However, there are several major disadvantages to the process which are: (1) the high energy consumption involved in concentrating the acid to 70% P.sub.2 O.sub.5, (2) the extreme difficulty in concentrating acid from the low quality phosphate rock, (3) the cost of large clarification equipment, and (4) the inability to remove impurities to very low levels. Further, if the acid is concentrated by either direct-fired concentration or by submerged combustion, atmospheric pollution becomes a problem.
Another general method of removing metal impurities from wet process acid involves the ammoniation of wet process acid. Metal impurities normally found in phosphate rock include MgO, Al.sub.2 O.sub.3 and Fe.sub.2 O.sub.3 as some of the more prominent impurities. Early efforts in this technique include the work of E. C. Houston et al as described in Ag and Food Chemistry, 3(1), 43-48, 1955. The process described involves the removal of impurities by the ammoniation of wet process acid. Ammoniation of the wet process acid (24.3% P.sub.2 O.sub.5) to a N/P.sub.2 O.sub.5 ratio of 0.37 results in the precipitation of Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3 and CaO impurities. No disclosure is made of MgO precipitation. The relatively high N/P.sub.2 O.sub.5 ratio makes processing the product into some finished fertilizer products both difficult and undesirable. Moreover, because the wet process acid is ammoniated to a relatively high initial pH or N/P.sub.2 O.sub.5 ratio, much valuable exothermic heat of reaction is lost and is not available for further processing of the acid.
Fitz-William, Jr. et al in U.S. Pat. No. 3,544,298 describe a process in which magnesium impurities can be removed from wet process acid. In the disclosed process superphosphoric acid (about 70% P.sub.2 O.sub.5) is ammoniated to a N/P.sub.2 O.sub.5 ratio of about 0.41. Upon cooling of the mixture to 60.degree. C., magnesium containing impurities precipitate. Subsequently, the solid and liquid phases are separated. The liquid phase is reduced to a MgO/P.sub.2 O.sub.5 ratio of 59.times.10.sup.-4 from a starting wet process acid which has a MgO/P.sub.2 O.sub.5 ratio of 145.times.10.sup.-4 ; this difference representing a 59% MgO reduction. However, no other impurities are disclosed as being precipitated by the process. Disadvantages of this process are the high costs of making superphosphoric acid and the problems of handling high N/P.sub.2 O.sub.5 weight ratio solutions.
The wet process acid treatment process disclosed by Moore et al in U.S. Pat. No. 3,554,728 is similar to that of Fitz-William, Jr. et al in that a superphosphoric acid (66-76% P.sub.2 O.sub.5) is ammoniated to a process solution having a N/P.sub.2 O.sub.5 ratio of 0.37-0.45. However, the reference then cools the solution to precipitate solids. After separation of the solids, the solution is made more acidic by adding crude wet process acid to the solution or by vaporizing ammonia from the solution to provide a liquid material having a reduced Mg content. Water is added to the solution to provide a liquid fertilizer product. However, the process has the disadvantage that a plurality of process steps are required thereby increasing costs. A significant drawback is that after the initial acid solution is ammoniated to a high N/P.sub.2 O.sub.5 ratio, the pH of the solution must be notably decreased by either vaporizing ammonia, thereby necessitating greater expenditures of energy, or by adding crude wet process acid to the solution which naturally will add impurities back into the acid.
Another prior art process disclosed by Knarr in U.S. Pat. No. 3,619,161 shows the clarification of wet process acid of carbonaceous impurities by treatment of the acid with a water-immiscible organic liquid. The reference does not show the precipitation of mineral impurities from wet process acid.
The Burch et al reference, U.S. Pat. No. 3,625,672, similar to other prior art processes discussed above, shows the precipitation of magnesium impurities from wet process acid by ammoniation of the acid to a high N/P.sub.2 O.sub.5 ratio or a high pH of 6.0 to 6.2. The low initial partial ammoniation process of the present invention is neither shown nor suggested by Burch et al.
The Tillman et al reference, U.S. Pat. No. 3,632,329 discloses a process specifically designed to remove magnesium impurities from ammonium phosphate fertilizer solutions by seeding the same with magnesium ammonium pyrophosphate. The reference does not show or suggest a process of precipitating mineral (magnesium) impurities by the direct ammoniation of wet process acid.
The Moore et al reference, U.S. Pat. No. 3,642,439, discloses a process of removing magnesium impurities from wet process acid by maintaining the HF and the soluble aluminum content at stated levels while concentrating wet process acid by evaporation of water to a concentrated acid of 45-53 wt % P.sub.2 O.sub.5. The magnesium containing impurities settle from solution while the solution is held at a temperature of 50.degree.-100.degree. C. The reference does not show the precipitation of magnesium containing impurities by the direct ammoniation of wet process acid.
The Mills reference, U.S. Pat. No. 4,136,199, discloses a method for removing metal ion impurities from wet process acid by adding to the acid, calcium and fluorine ions, which addition causes the precipitation of a fluoride ion containing solid, which solid also contains such metal ions as magnesium and aluminum. Nowhere, however, does the reference show the precipiation of impurities from wet process acid by the direct ammoniation of the same.
The Huber reference, U.S. Pat. No. 3,201,195, shows a method of precipitating substantially pure alkali metal and ammonium phosphate compounds from wet process acid and accordingly, as such is not concerned with the precipitation of metal ion impurities from wet process acid.
The Kenton reference, U.S. Pat. No. 3,926,610, and the Burkert et al reference, U.S. Pat. No. 3,630,711, relate to aspects of wet process acid treatment technology other than the direct ammoniation of wet process acid.
U.S. Pat. No. 4,325,927 (Charles W. Weston, et al) also discloses an impurity removal process by the ammoniation of wet process phosphoric acid. This patent teaches a two-stage ammoniation process. In the first stage, wet process phosphoric acid which has been obtained by diluting concentrated phosphoric acid is ammoniated to a pH of 1.5-2.5 and the reaction mass is aged for about 30 minutes. The pH is then raised to 4-5 by additional ammoniation and the solid and liquid phases are separated. A significant teaching of this patent is that the phosphoric acid being purified must have a low fluorine content; i.e., the phosphoric acid should contain no more than about 2.0% by weight fluorine. Under these conditions, a fluorine-free solid having the chemical composition (Al,Fe)NH.sub.4 (HPO.sub.4).sub.2.1/2H.sub.2 O is precipitated. Some disadvantages of this process are: (1) handling a liquid phase product at a high N/P.sub.2 O.sub.5 ratio which has a relatively low salting-out temperature; (2) the cost of first concentrating the phosphoric acid, diluting it for processing, and reconcentrating the final liquid phase; (3) the need to use a feed acid which has a relatively low fluorine concentration; and (4) producing a final liquid phase product which is unsuitable for direct conversion into an ammonium polyphosphate liquid fertilizer.
One technique for alleviating problems caused by impurity precipitation is based upon the addition of hydrogen fluoride to the ammonium polyphosphate liquid fertilizer product. This method is based on the phenomenon that the HF sequesters the impurities, rather than precipitating them. Accordingly, the impurities remain in the fertilizer product and are not removed. Since under some conditions the impurities can still precipitate from solution, the presence of the impurities in the fertilizer product constitutes a continuous potential disadvantage of using this technique. Moreover, the amount of plant nutrients present in the fertilizers produced is reduced proportionally by the amount of fluoride added. Other disadvantages are that excess fluoride ion in the fertilizer product can cause corrosion of aluminum storage tanks and can cause fluid fertilizer products to be unsuitable for use as an animal feed supplement. Still further, the large amounts of HF required by the process substantially increase fluid fertilizer production costs.
A need, therefore, continues to exist for a technique of simply removing metal impurities from wet process phosphoric acid before it is used to make phosphate-based fertilizers.